The demand for increasing areal density in the magnetic storage industry drives the search for new magnetoresistive readers with increased sensitivity. The tunneling magnetoresistive (TMR) read head is one device that has been investigated recently as a highly sensitive magnetoresistive reader. A TMR utilizes a magnetic tunnel junction (MTJ) composed of a tunnel barrier layer made from a non-magnetic insulating material sandwiched between two ferromagnetic layers. The insulating layer is thin enough to permit quantum-mechanical tunneling of charge carriers between the ferromagnetic layers. The tunneling is electron spin-dependent and, therefore, the tunneling current depends on the spin-dependent electronic properties of the ferromagnetic materials and the relative orientations of the magnetization directions of the ferromagnetic layers. For this reason, the two ferromagnetic layers are designed to have different responses to magnetic fields so that the orientation of their magnetic moments may be varied by an external magnetic field. One of the ferromagnetic layers in the MTJ, called the pinned layer, is composed of a material whose magnetic moment does not rotate in response to an applied magnetic field in the device's range of interest. In some MTJs the ferromagnetic layer is pinned by being exchange coupled to an antiferromagnetic layer. The other ferromagnetic layer is a free layer, that is, its magnetic moment is free to respond to an applied magnetic field in the device's range of interest.
Some MTJs include a tunnel barrier layer doped with magnetic particles. This doping provides an increase in magnetoresistance and an improved signal to noise ratio. For example, some MTJs have an aluminum oxide tunnel barrier layer doped with magnetic particles such as cobalt, iron or nickel particles.
The performance of MTJs depends to a large degree on the quality of the microstructures of the ferromagnetic and tunnel barrier layers. Compared with polycrystalline ferromagnetic layers, epitaxial ferromagnetic layers are highly desirable because they can reduce spin-flip scatterings and additionally enable one to control their crystal orientations to achieve high spin polarization yielding high magnetoresistive ratios. Unfortunately most MTJs presently available have only a single epitaxial ferromagnetic layer grown on an underlying substrate. Following the growth of this epitaxial ferromagnetic layer, conventional approaches to fabricating MTJs involve depositing a thin metal layer over the epitaxial ferromagnetic layer and exposing the metal to an oxidizing environment, such as air or pure oxygen, in order to oxidize the metal and form a thin tunnel barrier metal oxide layer. A second ferromagnetic layer is then grown over the tunnel barrier metal oxide layer. However, this second ferromagnetic layer is typically grown as a polycrystalline layer with random crystallographic orientations.
Other MTJs have a fully epitaxial structure where the first and second ferromagnetic layers as well as the tunnel barrier layer are all grown epitaxially on a substrate. Unfortunately, in order to have a lattice match between the tunnel barrier layer and the second ferromagnetic layer, only limited substrates are available. To date, no fully epitaxial MTJs have successfully been grown on a silicon (Si) substrate. For example, MTJs having an epitaxial MgO tunnel barrier oxide layer grown on an epitaxial iron (Fe) ferromagnetic layer have been developed. However, epitaxial growth of Fe cannot be carried out on a Si substrate. Therefore, the fully epitaxial MTJs presently available cannot take advantage of well established silicon processing techniques and have limited industrial applicability.
Thus, a need exists for a magnetic tunnel junction with two epitaxial ferromagnetic layers that may be grown on a variety of substrates, including silicon substrates.